Relationship between carbonized traces and space charges in oil-paper insulation

Among all the types of transformer insulation deterioration, partial discharge is relatively a direct one that may lead to carbon traces which have negative effect on insulation. We believe that the relationship between the trace length and total appearance charge of partial discharges is a key to diagnose the oil paper insulation´s remained life, even though the carbon trace in paper or pressboard cannot be seen directly like the transparent material such as XLPE. In this paper, we put forward an electrical method based on the hypothesis that the space charges just stay where the partial discharge happened before. Based on a distributed potential measuring system, we pay attention to carbon traces developed in a single electrical tree. After a series of high voltage experiments, we calculate the position of the space charges by their electric field and then we get the trace length by the farthest space charges. Finally, we get the result of experiment consisted with that of the theory calculations.